Generally, natural gas is liquefied and transported over a long distance in the form of liquefied natural gas (LNG). Liquefied natural gas is obtained by cooling natural gas to a very low temperature of about −163° C. at atmospheric pressure and is well suited to long-distance transportation by sea, since the volume thereof is significantly reduced, as compared with natural gas in a gas phase.
Even when an LNG storage tank is insulated, there is a limit to completely blocking external heat. Thus, LNG is continuously vaporized in the LNG storage tank by heat transferred into the storage tank. LNG vaporized in the storage tank is referred to as boil-off gas (BOG).
If the pressure in the storage tank exceeds a predetermined safe pressure due to generation of boil-off gas, the boil-off gas is discharged from the storage tank through a safety valve. The boil-off gas discharged from the storage tank is used as fuel for a ship, or is reliquefied and returned to the storage tank.
Examples of a marine engine capable of being fueled by natural gas include a dual fuel (DF) engine and an ME-GI engine.
A DFDE engine has four strokes per cycle and uses the Otto cycle in which natural gas having a relatively low pressure of about 6.5 bar is injected into a combustion air inlet, followed by pushing a piston upward to compress the gas.
An ME-GI engine has two strokes per cycle and uses a diesel cycle in which natural gas having a high-pressure of about 300 bar is injected directly into a combustion chamber in the vicinity of the top dead center of a piston. Recently, the ME-GI engine has attracted attention due to better fuel efficiency and propulsion efficiency thereof.